U.S. patent number 5,323,372 [Application Number 07/886,169] was granted by the patent office on 1994-06-21 for method of optical writing and reading on information carrier with high density storage.
This patent grant is currently assigned to Thomson-CSF. Invention is credited to Jean-Pierre Huignard, Brigitte Loiseaux, Michel Papuchon, Claude Puech.
United States Patent |
5,323,372 |
Puech , et al. |
June 21, 1994 |
**Please see images for:
( Certificate of Correction ) ** |
Method of optical writing and reading on information carrier with
high density storage
Abstract
A method for the optical writing of information elements uses a
material (Ma) that is highly non-linear optically, on which it is
possible to record information elements with sizes several times
smaller than the writing wavelength. Also disclosed is a method for
the reading of information elements recorded on a material (Ma)
deposited on a material (Mb) capable of being the site of a
stimulated emission generating the reading of the recorded
information elements. Applications: high definition television,
digital sound.
Inventors: |
Puech; Claude (Ballainvillers,
FR), Huignard; Jean-Pierre (Paris, FR),
Papuchon; Michel (Villebon, FR), Loiseaux;
Brigitte (Villebon sur Yvette, FR) |
Assignee: |
Thomson-CSF (Puteaux,
FR)
|
Family
ID: |
9412957 |
Appl.
No.: |
07/886,169 |
Filed: |
May 21, 1992 |
Foreign Application Priority Data
|
|
|
|
|
May 21, 1991 [FR] |
|
|
91 06111 |
|
Current U.S.
Class: |
369/116;
369/112.23; 369/275.2; G9B/11.012; G9B/11.016; G9B/11.049;
G9B/7.015; G9B/7.018; G9B/7.139; G9B/7.142; G9B/7.171 |
Current CPC
Class: |
G11B
7/00455 (20130101); G11B 7/005 (20130101); G11B
7/24 (20130101); G11B 7/243 (20130101); G11B
7/245 (20130101); G11B 11/10506 (20130101); G11B
11/10515 (20130101); G11B 11/10586 (20130101); G11B
7/252 (20130101); G11B 2007/24314 (20130101); G11B
2007/2431 (20130101) |
Current International
Class: |
G11B
7/252 (20060101); G11B 7/00 (20060101); G11B
11/00 (20060101); G11B 11/105 (20060101); G11B
7/24 (20060101); G11B 7/243 (20060101); G11B
7/005 (20060101); G11B 7/0045 (20060101); G11B
007/00 () |
Field of
Search: |
;369/100,109,116,118,275.1,275.2,112,121,122 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0343727 |
|
Nov 1989 |
|
EP |
|
0354601 |
|
Feb 1990 |
|
EP |
|
2308439 |
|
Dec 1990 |
|
JP |
|
2130423 |
|
May 1984 |
|
GB |
|
Other References
Journal of Applied Physics vol. 63, No. 11, Jun. 1, 1988, Woodbury,
N.Y., USA "Anomalous Electroabsorption in Semi-Insulating GaAs", L.
M. Walpita, pp. 5495-5499. .
Philips Technical Review vol. 42, No. 2 Aug. 1985, Eindhoven,
Netherlands "Erasable Magneto-Optical Recording", M. Hartmann, et
al. pp. 37-47..
|
Primary Examiner: Hannaher; Constantine
Assistant Examiner: Nguyen; Kiet T.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt
Claims
What is claimed is:
1. A method for optical writing of information elements comprising
the steps of:
generating a laser beam with a wavelength L;
focusing the laser beam on a surface of a non-linear optical
material to form the information elements on the non-linear optical
material;
wherein the steps of generating the laser beam and focusing the
laser beam on the surface of the non-linear optical material
determines that a maximum power density of the focused laser beam
is slightly greater than a threshold power density of the material
beyond which the material is optically transformed, so as to record
the information elements smaller than a focusing spot of the laser
beam.
2. The method for optical writing of information elements according
to claim 1, wherein the non-linear optical material is a
photosensitive resin.
3. The method for optical writing of information elements according
to claim 1, wherein the non-linear optical material is a
magneto-optical material.
4. The method for optical writing of information elements according
to claim 1, wherein the non-linear optical material is a metallic
material.
5. The method for optical writing of information elements according
to claim 1, wherein the non-linear optical material is a material
in which the variations in optical index being photoinduced.
6. The method for optical writing of information elements according
to claim 1, wherein the information elements have at least one
lateral dimension smaller than L/2.
7. The method for optical writing of information elements according
to claim 6, wherein the recorded information elements are
holes.
8. The method for optical writing of information elements
comprising the steps of:
generating a laser beam with a wavelength L;
focusing the laser beam on a surface of a non-linear optical
material to form the information elements on the material;
wherein a maximum power density of the focused laser beam is
slightly greater than a threshold power density of the material
beyond which the material is optically transformed, so as to record
the information smaller than a focusing spot of the laser beam;
and
wherein the non-linear optical material is deposited on a layer of
material generating a stimulated emission.
9. The method for optical writing of information elements according
to claim 8, wherein the non-linear optical material is metallic and
wherein the material generating the stimulated emission is a
polymethylmethacrylate type polymer doped with rhodamine.
10. The method for optical writing of information elements
according to claim 8, wherein the non-linear optical material is
metallic and wherein the material generating the stimulated
emission is a gallium arsenide type semiconductor.
11. The method for optical writing of information elements
according to claim 8, wherein the non-linear optical material is
metallic and wherein the material generating the stimulated
emission is a glass or a crystal doped with a rare earth.
12. A method for optical reading of information elements on an
information carrier, the information carrier comprising two
superimposed layers of a first material and a second material, said
second material generating stimulated emission when illuminated
beyond a threshold power density, comprising the steps of:
generating a laser beam with a wavelength L;
focusing the laser beam on the information carrier;
wherein only first regions of the first material in which the
information elements have been recorded are transparent to the
laser beam;
wherein the focusing of the laser beam at the wavelength is done on
a surface of second regions of the second material that are facing
the first regions; and
wherein the laser beam has a power such that a power density at a
center of the laser beam exceeds the threshold power density of the
second material solely in a small zone with a surface area that is
several times smaller that the surface area of the focusing spot of
the laser beam, a presence or absence of the stimulated emission of
the second material allowing for detecting the information
elements.
13. The method for optical writing of information elements on an
information carrier according to claim 12, wherein the first
material is metallic and the first regions are holes.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the storage of information by
optical means, as well as to the reading of this information.
In technologies for the optical storage of information, the maximum
density of storage is limited by the diffraction at the wavelength
of the writing or of the reading. For the optical wavelengths
commercially available in the form of laser diodes, this limit may
be, for example, in the region of one bit per square micrometer.
With the new blue laser sources emitting towards 0.4-0.5 .mu.m, it
becomes possible to increase the storage of the information
accordingly. Typically, an increase in storage density by a factor
of 4 to 5 is expected as compared with current sources emitting in
the near infra-red range. However, even these storage systems do
not appear to be likely to enable surface densities of information
suited to the storage of high definition television signals of a
duration sufficient for large-scale consumer applications.
In this context, systems based on storage in volume have been
proposed more recently. These systems should provide a gain by a
factor of several tens in terms of surface density of information
elements, but with the drawback of substantially increased
complexity.
This is why the present invention proposes a new method of optical
writing capable of giving rise to information carriers with high
density of surface storage. The present invention also proposes a
method for the reading of an information carrier with high density
of surface storage.
2. Description of the Prior Art
At present, the approaches that enable surface storage are limited
by problems of diffraction.
Indeed, when a writing laser beam is focused on the surface of a
carrier, it is not focused at one point but in a region in which
the distribution of density of power is the one shown in FIG. 1a.
It is defined by the following surface equation:
with Z=2
where
J1 is the first order Bessel function
and
x and Y are surface coordinates, and
where
L is the wavelength of the incident beam.
A is the angle defining the focus of the lens used for the
convergence of the beam (this angle is shown in FIG. 1b). It is the
angle of aperture of the focusing lens.
Thus the focusing spot, called the Airy spot, has a base is defined
by a radius R.sub.o with
These relationships are valid in the case of a uniform illumination
of the pupil of the objective and for a less chromatic radiation.
In practice, since the laser beam has a Gaussian distribution, the
illumination in the pupil of the objective has an intensity with
the shape of a truncated Gaussian curve. Furthermore, it has a
certain spectral width. The distribution of illumination at the
focus of the objective is not exactly an Airy function, but the
approximation made herein is considered to be representative of the
real phenomenon.
In the extreme case of a maximum focusing aperture (A=90.degree.),
R.sub.om =1.22 L/2. However, for a smaller aperture, hence for a
smaller field depth, the base of the Airy spot has a radius greater
than R.sub.om. The radius R.sub.om corresponds rather to the size
of the spot at mid-height of the curve D(x,y) and enables the
definition of the size of an information element recorded with a
sufficient density of power. Thus, typically, in using a recording
beam focused at the wavelength L, it is not possible to record
information elements having a lateral size smaller than L.
The present invention proposes the use of a material that is highly
non-linear optically (Ma) and a focused laser beam having a maximum
power density Pmax such that the threshold power density Pthreshold
of the material (Ma) is slightly lower than Pmax. FIG. 2 shows that
the invention uses the upper part of the curve D(x,y). It is
concerned with the regions in which a small variation of the
density of recording power leads to corresponding reductions at the
level of the surface of the recorded information elements. The
optical response of the material (Ma) should therefore be as highly
non-linear as possible and the densities of power P.sub.max and
P.sub.threshold are matched in such a way as to be on either side
of the bending point of the curve illustrating the optical response
of the material (Ma) with the density of power that it receives.
This curve is shown schematically in FIG. 3. The hatched part of
FIG. 2 corresponds to zones in which the material is transformed
optically and which define the regions in which there has been a
recording of information elements.
The size of the information elements may thus be far smaller than
in the prior art and, hence, for an equivalent storage surface
area, the storage density is notably increased.
SUMMARY OF THE INVENTION
More specifically, the invention proposes a method for the optical
writing of information elements wherein:
it uses a laser with a wavelength L focused on the surface of the
material (Ma) that is highly non-linear optically;
the maximum power density of the focused laser beam is slightly
greater than the threshold power density of the material (Ma)
beyond which the material (Ma) can get transformed optically so as
to record an information element that is appreciably smaller than
the focusing spot of the beam used.
In other words, the power of the laser is chosen so that the power
density in the laser beam exceeds the threshold power density only
in a zone with a surface area that is several times smaller than
the surface area of the focusing spot of the beam (for example, at
least three times smaller).
Preferably, the material (Ma) may be a photosensitive resin or a
magneto-optical material or a metallic material or a material with
phase transition.
An object of the invention is also the carrier of information
recorded by the above-described writing method, the information
carrier being one wherein, if L is the wavelength of the writing
beam, then the recorded elements have at least one lateral
dimension smaller than L/2, these information elements being
possibly holes. In the carrier according to the invention, the
material (Ma) may be deposited on a material (Mb) capable itself of
generating a light radiation by stimulated emission.
In the prior art, the wavelengths used are such that it is possible
neither to write very small-sized information elements (smaller
than L if L is the writing wavelength) nor even to read the very
small-sized information elements which would have been written by
other methods for reasons related to the focusing of the reading
beam and to "smearing".
This is why another object of the invention is a method for the
optical reading of a very small-sized information carrier. This
method uses a carrier comprising two superimposed layers of
material (Ma) and material (Mb), the material (Mb) being a material
capable of stimulated emission when it receives a power
only the regions (Ra) of the material (Ma) in which information
elements have been recorded are transparent to the reading
wavelength L1;
the focusing of the reading beam at the wavelength L1 is done on
the surface of the regions (Rb) of the material (Mb) that are
facing the regions (Ra);
the reading laser beam has a power such that the power-density at
the center of the beam exceeds the threshold power solely in a
small zone with a surface area that is several times smaller than
the surface area of the focusing spot of the laser beam.
The detection of information is done by the detection of the
presence or absence of a stimulated emission.
The material (Ma) used may be metallic, the regions (Ra) may be
holes and the material (Mb) may equally well be a semiconductor or
a polymer doped with a dye or a glass or a crystal doped with rare
earths.
Finally, an object of the invention is the carrier for optical
storage capable of being read by the reading method according to
the invention, the carrier comprising at least two layers of
materials deposited on a substrate, the upper layer serving as a
carrier for the recording of information, the lower layer
representing a medium capable of emitting light by stimulated
emission during the reading.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention shall be understood more clearly and other
advantages shall appear from the following description and from the
appended figures, of which:
FIG. 1 shows the distribution of power density of a laser beam
focused by a lens (L);
FIG. 1a shows the shape of the curve of distribution as a function
of one of the dimensions x and y of the surface area on which there
is focusing;
FIG. 1b defines the angle A corresponding to the focus of the lens
L, this angle playing a direct role in the determining of the base
of the curve shown in FIG. 1a;
FIG. 2b enables the locating, on a curve of power density of a
focused beam, of the threshold power density beyond which the
optically non-linear material (used in the invention) gets
transformed;
FIG. 3 shows a schematic view of the optical response of the
material (Ma) as a function of the power density that it
receives;
FIG. 4 illustrates the highly non-linear optical response of a
photosensitive resin that can be used in the invention;
FIG. 5 shows the hysteresis cycle described by magnetization of a
magnetic material that can be used in the invention as a function
of the field applied;
FIG. 6 illustrates an example of a device enabling the recording of
information elements on a plane disk;
FIG. 7 illustrates an exemplary reading device according to the
invention.
MORE DETAILED DESCRIPTION
The present invention proposes a new method of optical writing on a
carrier comprising a layer of material (Ma), the optical response
of which is highly non-linear; the material (Ma) may be a
photosensitive resin. The chemical development of which may enable
a submicronic resolution to be achieved. The response of certain
photosensitive resins may be more highly non-linear as shown in
FIG. 4. This is a Shipley S1400 resin, developed with a developer
MF 314, To being the initial thickness and T being the thickness
after insolation. The curve shown in FIG. 4 shows the progress of
the ratio T/To in terms of percentage as a function of the density
of energy received by the resin. The behavior of this material is
particularly promising for the defining of a power density of the
focused writing laser beam close to the threshold power density of
the photosensitive resin used.
The material (Ma) may be a material in which a variation of
refraction index can be photoinduced. One of the promising features
of materials such as these is that they can record an information
element in optical form without going through a stage of chemical
development. They may be, for example, photopolymer materials.
Photosensitive resins or photopolymers can easily be deposited on
any substrate by the centrifugation method. The thickness of the
layer of material (Ma) can be easily adjusted by determining the
centrifugation speed and by determining the concentration of the
initial solution of material (Ma).
The material (Ma) may also be a magneto-optical material. Under the
effect of a laser beam, the magneto--optical material gets heated
up, the local rise in temperature leads to variations of the
magnetic parameter and the information bit is defined by the zone
in which the previously oriented magnetization takes an
antiparallel direction. More specifically, the layer of material
(Ma) may be brought to saturation beforehand by the application of
an external magnetic field greater than the coercive field at the
ambient temperature (the coercive field Hc corresponds to the field
capable of cancelling the induced magnetization and is illustrated
by the hysteresis cycle of FIG. 5 giving the development of the
magnetization J with the magnetic field H). Then an external field
is applied in the plane of the layer, this external field being
weaker than the coercive field and having a direction opposite that
of the saturation field. When the heating prompted by the laser
brings the material (Ma) beyond the Curie temperature Tc (the
temperature at which the behavior of the material becomes
paramagnetic), the magnetization flips over to an antiparallel
direction, thus defining a bit. The transformation of the
magneto-optical material therefore occurs only when the power
density that it receives is greater than a certain threshold power
density corresponding to a heating of the material such that the
latter is taken to a temperature greater than the Curie temperature
Tc.
The layers of magneto-optical materials may be made of cobalt
combined with chromium or nickel or, again, of phosphorus. The
manganese-bismuth material also has promising magnetic qualities
owing to the hexagonal system in which it crystallizes. Table I
illustrates exemplary embodiments of carriers made of
magneto-optical material, the deposition method of which depends on
the nature of the material.
TABLE I
__________________________________________________________________________
MATERIAL THICKNESS SUBSTRATE MANUFACTURE
__________________________________________________________________________
Chromium-cobalt 80 Glass+gold Vacuum deposition Chromium-phosphorus
60 Glass+gold Electrolysis Manganese-Bismuth 70 Glass Vacuum
deposition
__________________________________________________________________________
Irrespectively of the material (Ma) deposited on its substrate, the
recording of information can be done by a laser with a short
wavelength (for example a blue argon laser emitting at 457.9 nm) or
a coherent source obtained by frequency doubling of a semiconductor
laser or a neodymium type YAG laser focused on the surface of the
layer of material (Ma). By appropriately adjusting the maximum
power density of the optical beam focused, it is possible to record
information elements smaller than 0.2 .mu.m.
The recording method according to the invention can be carried out
on carriers in the form of disks, cards (credit card type) or else
tapes.
FIG. 6 illustrates an example of a device enabling the recording of
information elements on a plane carrier disk capable of being made
to rotate about an axis.
For the recording done by a laser beam (F) focused by means of a
focusing objective (OF) on the layer of material (Ma) deposited on
a substrate (S), the focused laser beam generates a focusing spot
(T), only one central part (C) of which creates a transformation of
the material corresponding to the information recorded. The
transformed zone (ZT) has lateral dimensions several times smaller
than those of the spot (T). This entails the assumption that the
power density at the center of the spot should be slightly greater
than the threshold power density of reaction of the material, for
example it should be greater by a maximum of 20%. The focusing
optical system can be shifted radially in relation to the disk (in
a standard way) so that the entire useful surface of the disk can
be scanned by the recording laser beam. The modulation of the
recording beam according to the signal to be recorded on the
carrier can be obtained in many ways. In a first embodiment, a
light modulator (Pockels cells or other) is interposed on the path
of the laser beam. This modulator (MO) is controlled so as to let
through or interrupt the beam according to the information to be
recorded. In a second embodiment, the laser itself may be modulated
(it may be a semiconductor laser in particular with modulation of
the injection current). Indeed, the information elements may be of
a binary type, each recording by the useful part of the focused
laser beam representing an information bit, or else the information
elements may be of an analog type and, in this case, the useful
information is represented by the length of a zone modified by the
laser beam or again the distance between two successive local
modifications of the layer of material (Ma).
A synchronization is provided between the modulation control and
the relative shift of the disk and of the focusing spot so that the
information elements are recorded with a well-determined spatial
distribution. By recording information elements, one of the lateral
dimensions of which is smaller than a recording half wavelength, it
becomes also possible to obtain a spacing between recorded dots
smaller than a half wavelength.
By making the most efficient possible adjustment of the maximum
power density of the focused laser beam and the threshold power
density of the material (Ma), it becomes possible to record
information elements having sizes of the order of 0.1 .mu.m
separated by an equivalent distance of the order of 0.1 to 0.15
.mu.m. A configuration such as this makes it possible to obtain a
surface density of 2.10.sup.9 to 2.5.10.sup.9 bits/cm.sup.2.
A surface density of information elements of this kind makes it
possible to envisage optical recording in the field of high
definition television.
Indeed, assuming that it is necessary to have a bit rate of 250
Mb/s, a one-hour television program corresponds to about 10.sup.12
bits, namely an area of 400 to 500 cm.sup.2. A disk with a diameter
of 30 cm, the useful surface area of which is between R.sub.1 =15
cm and R.sub.2 =3 cm, provides a useful surface area of 675
cm.sup.2. It becomes possible to store an HDTV program of 80
minutes to 100 minutes per side. However, to make it easy to obtain
freezing, slow motion and fast motion, it may be desirable to make
an image correspond to a whole number of turns. Given the linear
density, there should be ten tracks on the internal radius. If this
arrangement (10 tracks/image) is preserved on the entire 30 cm
disk, the duration of the program becomes equal to 40 minutes per
side.
The use of an information carrier with such surface densities of
information may also be promising in the field of digital sound.
Indeed, assuming that it is necessary to have 10.sup.2 kbits/s
available to store digital sound, the useful surface area to store
one hour of digital sound becomes equal to 0.2 cm.sup.2. A
mini-compact disk type configuration, with a diameter of several-
centimeters, may enable the storage of programs of several
hours.
At present, the reading of very small-sized information elements
very close to each other imperatively calls for the use of sources
with small wavelengths (for example a blue source) in order to
avoid, to the maximum, the simultaneous reading of several
information elements due to the width of the focusing spot of the
reading beam. This is why the present invention makes the maximum
use also of the non-linearity of certain materials for the reading
of information elements so that only the central part of the
focusing spot of the reading beam can read information elements
with sizes smaller than that of the focusing spot. It becomes
possible with the reading method according to the invention, with a
reading beam emitted at L, to read information elements having
sizes appreciably smaller than L. In the case of information
elements recorded according to the writing method of the invention,
the wavelength of the reading beam may even be greater than the
wavelength of the reading beam.
Thus, for the reading of an information carrier according to the
invention, or of any other information carrier stored with very
high surface densities, the invention proposes the coupling of the
layer of material containing the information to a layer of material
capable of being the site of stimulated emission. Indeed,
stimulated emission phenomena intrinsically display non-linear
behavior Hence, a material (Ma) is chosen wherein the previously
recorded information elements are transparent to the wavelength
L.sub.1 of the reading beam which, in irradiating the material
(Mb), generates a stimulated emission at the wavelength L.sub.2
only very locally at the place where the power density of the beam
at L.sub.1 is greater than the threshold power density beyond which
there is stimulated emission. The material (Mb) may be varied in
nature. It may be a polymer doped with rhodamine for example.
Indeed, the threshold power density of such a material is in the
range of 100 kW/cm.sup.2. A laser emitting at the wavelength
L.sub.1, at a few mW, may get focused on a light spot with a size
of 0.5 .mu.m and is capable of prompting a stimulated emission
making it possible to reveal the information elements stored in the
material (Ma).
The material (Mb) may also be a gallium arsenide type of
semiconductor emitting in the near infrared range.
The material (Mb) may also be constituted by a glass or a crystal
doped with a rare earth (neodymium in particular) and capable of
emitting a radiation in the near infrared range.
The choice of the reading laser is dictated by the nature of the
material (Mb) chosen. Its wavelength should be located in the
absorption bands of the material Mb so as to induce the emission by
the material Mb, at a different wavelength, generally greater than
that of the reading laser. The reading method is thus far different
from the reading methods of the optical carrier known to those
skilled in the art, inasmuch as the reading is obtained not by
diffraction of the reading beam by the information elements, but by
light emission of the carrier itself at a wavelength different from
that of the reading beam.
The reading may also be carried out by a frequency doubler material
emitting at L1/2. At present, there are polymer type organic
materials having good non-linear optical properties under frequency
doubling. These materials are especially advantageous in terms of
implementation and cost.
FIG. 7 shows an example of a device using an information carrier
according to the invention. This carrier is constituted by a layer
of metal and the stored information elements are holes in this
layer. This layer is superimposed on a film of polymer (Mb) of the
polymethylmethacrylate type doped with rhodamine, this film of
material (Mb) being itself deposited on a transparent substrate.
The reading method is carried out in transmission, a focusing beam
is focused by means of an objective (OF) on the polymer film, the
metal elements that are not etched at recording acting as masks-
for the incident reading beam emitting at the wavelength L.sub.1.
Only the regions Rb of the material (Mb) that are facing the
regions Ra between two metal pads in the present case receive the
totality of the incident beam emitting at L.sub.1. Furthermore, the
power density received in a region Rb is such that only the regions
R'b of the regions Rb receive a power density sufficient to
generate a stimulated emission at the wavelength L.sub.2. The
regions of the material (Ma) that are different from the regions
Ra, although they receive incident reading beam, transmit only a
focused power density that is smaller than the threshold power
density of the material (Mb), and cannot thus generate a stimulated
emission at the level of this material. This reading method can be
used to discriminate between one information element and a
neighboring information element, even if these information elements
are very small-sized (smaller than 0.2 .mu.m) and separated by a
very small distance (smaller than 0.2 .mu.m).
A filter letting through only the emission wavelength L.sub.2 can
be positioned at output of the substrate. The filtered signal can
be collected by a photodiode so as to restore the stored
signal.
* * * * *